There exist various techniques for measuring and accurately positioning laser beams. The following early publications represent the general state of the prior art:
"Laser Recording Performance With Spatial Error Compensation", S. Bousky and L. Teeple, SPIE, Vol 53, 1973. In this paper, a polygon deflector is employed together with a reference line for determining cross scan error. Two-dimensional corrections are made to the beam position, which is measured differentially.
"Beam Deflection at High Accuracy and Precision", D. P. Jablonowski and J. Raamot, SPIE, Vol 84, 1976. This paper describes X-Y deflection using galvanometer-driven mirrors or polygons, using crossed gratings to measure two-dimensional position information. A reference beam is deflected to measure position and is used to measure the beam position in two dimensions during writing.
"Ultrahigh Resolution Data Terminal", M. R. Smith, R. H. Burns and R. C. Tsai, SPIE, Vol. 200, 1979. This paper describes a high resolution display that uses a laser to address a liquid crystal gate. Deflection is performed by a galvanometer-driven mirror. A reference beam hits a crossed Ronchi grating and the resulting information is used to instantaneously determine the two-dimensional coordinates of the beam.
One of the main problems encountered in high-quality laser beam scanning relates to accurate positioning of a laser generated dot relative to adjacent dots, as noted in the literature (e.g. Bestenheimer et al. in Journal of Appl. Phot. Eng. vol. 2, 1976). The human eye has extreme sensitivity to periodic errors in the interline separation of beams. Periodic errors resulting from intensity changes, or positional fluctuations, can lead to banding of high visibility. Passive and active methods have been used to minimize these errors.
Various laser scanning devices are known in the art. They typically comprise a laser source and optical elements for bringing the laser light produced by the laser source to a scanning medium.
U.S. Pat. No. 4,746,180 to Deisler et. al. describes a laser scanning device composed of at least one multi-mode fiber optical light lightguide and one guide element for the fiber optical lightguide. As is known in the art, multi-mode fiber optical lightguides will produce inhomogeneous light distributions on an exit face in the face of a change of curvature, typically due to motion of one portion of the lightguide. Thus, the device of U.S. Pat. No. 4,746,180 utilizes a rigid guide element to ensure that the curvature of the multi-mode fiber optical lightguide is maintained during movement of the exit face of the lightguide. It is a further purpose of the present invention to utilize fiber optical lightguides which can move freely while maintaining a constant light distribution at their exit faces.